1. Field of the Invention
The present invention relates to a method and apparatus for driving an ink jet recording head having piezoelectric units and, in particular, a method and apparatus for driving an ink jet recording head to provide uniform ink jetting characteristics of the ink jet recording head without being influenced by fluctuations in temperature of the surrounding environment.
2. Description of the Related Art
FIGS. 1(a) and 1(b) illustrate a conventional on-demand type ink jet recording head for an ink jet printer. In particular, FIG. 1(a) is a sectional view of an actuator of the ink jet recording head, and FIG. 1(b) illustrates an electrical equivalent circuit for the same.
An ink chamber 2 of the actuator is formed of an ink chamber frame 1 and a vibrating plate 4, and a piezoelectric unit 5, which expands and contracts as controlled by an electric field applied thereto, is rigidly mounted on the vibrating plate 4. The volumetric capacity of the ink chamber 2 is expanded or contracted as the vibrating plate 4 is displaced by operation of the piezoelectric unit 5.
To impose an electric field on the piezoelectric unit 5, an electric voltage is applied from an outside power source via driving line 7 to an electrode 6 which is disposed on the piezoelectric unit 5. This applied voltage causes a distortion in the piezoelectric unit 5, and this distorting force causes the vibrating plate 4 to exert an abrupt pressing force on the ink chamber 2, thereby jetting forth ink droplets 3 out of very small holes present in the ink chamber frame 1. A plurality of actuators such as this are disposed in the an ink jet recording head.
The piezoelectric unit 5 can be represented in electrical terms as a capacitor, as shown in FIG. 1(b). Accordingly, electric current in proportion to the time differential of the waveform of the applied voltage flows as a charging current and a discharging current when the electric field is applied and removed, respectively.
As the voltage is applied to and removed from the piezoelectric unit 5 at a higher frequency, the charging current and the discharging current increases. Hence, when a large number of driving elements are present in the piezoelectric unit 5, for example, when 24 elements are driven, the charging or discharging current will have a peak value of 50A. Consequently, a large drop in the voltage on the driving line 7 occurs. This results in a considerable change in the jetting characteristics effected by the driving elements in the piezoelectric unit 5, and also a breakdown of electronic components in the piezoelectric unit 5.
Therefore, in an attempt to eliminate these problems, a method for driving a piezoelectric unit exists in which the charging and discharging currents are limited by current limiting resistors. This method is described in U.S. Pat. Nos. 4,459,599, 4,126,867, and 4,282,535.
In this method, however, the resistance values of the current limiting resistors are readily susceptible to change. As a result, the charging and discharging times for the individual piezoelectric units vary, which affects the ink jetting characteristics considerably. This problem also exists in integrated circuit (IC) including such current limiting resistors.
Moreover, the charging and discharging times of the piezoelectric units will eventually become different due to inherent changes in capacitance of the individual piezoelectric units. Hence, because of this, the ink jetting characteristics will deteriorate even if current limiting resistors having identical resistances could formed in an IC with a high degree of accuracy.
As an alternative, U.S. Pat. No. 4,284,996 describes a method for driving one piezoelectric unit with constant current sources, one of which being a flow type and the other being a synchronizing type. However, in this method, it is difficult to maintain uniform driving characteristics when driving a plurality of piezoelectric units due to the differences in the capacitances of the piezoelectric units and the differences in the current values of the constant current sources.
In an attempt to eliminate the problems associated with the conventional apparatuses, such as those described above, applicant has previously developed a driving device which is capable of suppressing the peak values of the charging and discharging currents without being influenced by the differences in the capacitance of the piezoelectric units, and without using any current limiting resistors. In particular, in the driving device, a plurality of piezoelectric units are directly coupled to a driving power source whose output voltage is fluctuated at a prescribed gradient in a single driving period for the piezoelectric units. This device is described in Japanese Laid Open Patent Application Nos. 274554-1990 (Heisei 2), 36036-1991 (Heisei 3), 133647-1991 (Heisei 3), 164544-1990 (Heisei 2), 369543-1992 (Heisei 4).
FIGS. 2(a) and 2(b) of the present application illustrate this driving device. A high voltage Vp of approximately 20 V is input to the driving voltage generator 10 and 20, while a Vd of approximately 5 V is input to power the logic circuit. The driving voltage generators 10 and 20 are grounded at GND, and output a driving voltage Vs for driving the piezoelectric units 14. In subsequent figures, identical parts are identified by the same numbers or characters.
The driving circuit of FIG. 2(a) is employed in an ink jet recording head which uses d31 type piezoelectric units and has pressure chambers which expand upon application of a voltage to the piezoelectric units, thereby sucking ink out of the reservoir and the pressure chamber upon removal of the voltage and thus jetting out ink droplets. In this driving circuit, the individual piezoelectric units 14 of the piezoelectric unit array 13 are connected, via bi-directional transfer gates 12 acting as selecting switches in a block of selecting switches 11, to the output of a driving voltage generator 10, which generates the driving voltage Vs that changes at a desired gradient. Because the electric power is applied selectively to these transfer gates 12 to charge and discharge the piezoelectric units 14 which are made of the same base material, the driving voltage Vs at the prescribed voltage gradient is applied selectively to the piezoelectric units 14 without being influenced by any dispersion in their capacitance.
FIG. 2(b) illustrates a driving circuit which is employed in an ink jet recording head which uses d33 type piezoelectric units and has pressure chambers which expand upon application of a voltage to the piezoelectric units, thereby sucking ink out of the reservoir and the pressure chamber upon removal of the voltage and thus jetting out ink droplets. The driving voltage generator 20 generates a negative driving voltage -Vs which changes at a desired gradient.
In this driving circuit, a common terminal of the piezoelectric units 14 of the piezoelectric unit array 13 is connected to the output of driving voltage generator 20 which generates the driving voltage -Vs that changes at the desired gradient. The other ends of the piezoelectric units 14 are connected to ground GND via a group of selecting switches 21 comprising mono-directional transistors 22, each having a parasitic diode 23 coupled in parallel. These selecting switches can be formed in an IC, such as those presently sold on the market.
When the driving voltage -Vs grows larger at a constant gradient in the negative direction, all the piezoelectric units 14 are charged via the parasitic diodes 23, and the pressure chamber is thereby expanded. Then, as the driving voltage Vs becomes smaller (i.e., is reduced toward zero) at a prescribed gradient, electric power is conducted through only transistors 22, which have a signal being applied to their bases sufficient to turn on the transistors. Hence, the piezoelectric units 14 corresponding to those transistors 22 are discharged, which causes the corresponding pressure chambers to contract, thereby jetting out the ink. Moreover, the transistors 22 having an "off" signal (i.e. a voltage not large enough to turn on the transistor) applied to their bases remain non-conductive, and thus, the piezoelectric units 14 coupled to these transistors maintain their charged state, and do not jet out any ink.
FIG. 3(a) illustrates an arrangement of the circuit of the driving voltage generator 10, which generates a driving voltage Vs, as shown in FIG. 2(a). This driving voltage generator 10 charges a capacitor C via a transistor TR1 at a time constant determined by the capacitance of capacitor C and the resistance of resistor Rt1, and thereafter discharges the capacitor C via a transistor TR2 at a time constant determined by the capacitance of capacitor C and the resistance of resistor Rt2, thereby obtaining a reference fluctuating voltage. The driving voltage Vs is generated by amplifying this fluctuating voltage of the capacitor C by a power amplifier 30. This power amplifier 30 comprises a pair of mutually complementary transistors.
This circuit illustrated in FIG. 3(a) is simple and low cost. However, since the waveform has the characteristics of an exponential function, the waveform tapers near the end of operation of the actuator. Hence, this driving device cannot attain favorable ink jetting characteristics. Moreover, the charging and discharging currents of the piezoelectric unit, respectively, have the characteristics of an exponential function and thus do not provide constant current. Accordingly, in this circuit, large current flows in the initial phases of the charging period and discharging period, which is not favorable.
FIG. 3(b) illustrates a driving voltage generator which overcomes the defects of the conventional driving devices described above. This driving device attains a reference fluctuating voltage by charging and discharging the capacitor C with constant current sources 31 and 32. The reference fluctuating voltage is amplified by the power amplifier 30, and the driving voltage Vs is thereby obtained.
The piezoelectric units are thus driven with constant charging and discharging currents at the driving voltage Vs. When this process is used, the voltage Vc of the capacitor C will be Vc=(i1/C).times.t or Vc=Vp-(i2/C).times.t (wherein, t: time). The voltage Vc, therefore, undergoes a linear change, and thus, this voltage generator overcomes the problems found with the conventional driving voltage generators.
FIGS. 4(a) through 4(c) illustrate the wave-forms provided by the driving voltage generator, which attains a linear change in the generated voltage, as shown in FIG. 3(b). In particular, FIGS. 4(a) and 4(b) show the wave-form of the driving voltage Vs that is provided, and FIG. 4(c) shows the wave-form of the driving current Is that is provided. Actually, FIGS. 4(a) and 4(b) show the driving voltage Vs that it is used for the d31 type piezoelectric units and the d33 type piezoelectric units, respectively.
Differences in the base material of which a piezoelectric unit is made, such as a difference in the thickness of the base material, will result in a difference in the amount of expansion or contraction of the piezoelectric unit, so that there will be differences in the quantity of the jetted ink, the velocity of the jetted ink, and so forth. Accordingly, it is necessary to adjust the value of the wave height of the driving voltage Vs in order to compensate for such differences.
Furthermore, it is necessary to adjust the value of the wave height of the driving voltage when there is a sharp change in the characteristics (e.g. viscosity) of the ink in relation to the temperature in the environment. Such a correction can be made by adjusting the charging and discharging times of the piezoelectric unit, as shown by a single dot chain line in FIGS. 4(a) and 4(b). However, such an adjustment increases the time in which the ink chamber is maintained in its state of its maximum expansion from t0 to t0'. As a result, a deviation occurs in the position of the meniscus of the ink at the point in time when the ink is jetted out, and thus, a change eventually occurs in the ink jetting characteristics.
Also, with the driving voltage generator shown in FIG. 3(b), the charging current i1, which is fed from the constant current source 31 to the capacitor C, and the discharging current i2, fed from the constant current source 32, will assume the values determined by the following equations: EQU i1=Ebe/Rs2 EQU i2=Ebe/Rs4
where the voltage between the base and emitter in transistors TR3 and TR6 is expressed as Ebe.
However, the voltage between the base and emitter of a transistor depends on the temperature in the environment and changes considerably from 0.7 V to 0.35 V in the range from 0.degree. to 40.degree. C. Hence, the circuit shown in FIG. 3(b) suffers from problems related to the temperature characteristics.